Pulse width modulation (PWM) motor drive devices that aim to supply motor drive current efficiently employ soft switching driving that enables the motor to be driven noiselessly and efficiently by varying the PWM duty ratio to gently change the motor current when the polarities of motor current are switched in accordance with a polarity of the motor.
A conventional technique for gently changing fluctuations of motor current has been disclosed (refer to PTL 1). More specifically, a PWM motor drive device counts a period of a motor phase signal indicating a polarity of a motor and then allocates soft switching zones to the counted period, thereby setting the soft switching zones based on the cycle of the motor. As a result, when the PWM motor drive device switches the polarities of the motor current, an inclination of a slew rate decreases.